U.S. patent application number 11/302724 was filed with the patent office on 2006-10-05 for mode-locked semiconductor lasers with quantum-confined active region.
Invention is credited to Allen L. Gray, Hua Huang, Hua Li, Petros M. Varangis, Lei Zhang, John L. Zilko.
Application Number | 20060222024 11/302724 |
Document ID | / |
Family ID | 37070417 |
Filed Date | 2006-10-05 |
United States Patent
Application |
20060222024 |
Kind Code |
A1 |
Gray; Allen L. ; et
al. |
October 5, 2006 |
Mode-locked semiconductor lasers with quantum-confined active
region
Abstract
A mode-locked integrated semiconductor laser has a gain section
and an absorption section that are based on quantum-confined active
regions. The optical mode(s) in each section can be modeled as
occupying a certain cross-sectional area, referred to as the mode
cross-section. The mode cross-section in the absorber section is
larger in area than the mode cross-section in the gain section,
thus reducing the optical power density in the absorber section
relative to the gain section. This, in turn, delays saturation of
the absorber section until higher optical powers, thus increasing
the peak power output of the laser.
Inventors: |
Gray; Allen L.;
(Albuquerque, NM) ; Huang; Hua; (Albuquerque,
NM) ; Li; Hua; (Albuquerque, NM) ; Varangis;
Petros M.; (Albuquerque, NM) ; Zhang; Lei;
(Albuquerque, NM) ; Zilko; John L.; (Albuquerque,
NM) |
Correspondence
Address: |
FENWICK & WEST LLP
SILICON VALLEY CENTER
801 CALIFORNIA STREET
MOUNTAIN VIEW
CA
94041
US
|
Family ID: |
37070417 |
Appl. No.: |
11/302724 |
Filed: |
December 13, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
60662451 |
Mar 15, 2005 |
|
|
|
60723412 |
Oct 3, 2005 |
|
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|
Current U.S.
Class: |
372/18 ;
372/45.013 |
Current CPC
Class: |
H01S 5/1014 20130101;
H01S 5/22 20130101; H01S 5/1064 20130101; H01S 5/34 20130101; H01S
5/3412 20130101; H01S 5/065 20130101; H01S 5/10 20130101; H01S
5/0602 20130101; B82Y 20/00 20130101; H01S 5/0657 20130101 |
Class at
Publication: |
372/018 ;
372/045.013 |
International
Class: |
H01S 3/098 20060101
H01S003/098; H01S 5/00 20060101 H01S005/00 |
Claims
1. An integrated mode-locked semiconductor laser for producing
laser pulses comprising: a horizontal laser cavity integrated on a
semiconductor substrate, the laser cavity having an optical path; a
quantum-confined active region located along the optical path; a
gain section including a first portion of the quantum-confined
active region; an absorber section including a second portion of
the quantum-confined active region, wherein a mode cross-section of
the absorber section has a larger area than a mode cross-section of
the gain section, and the gain section and/or the absorber section
produce a loss modulation applied to laser pulses propagating
around the laser cavity.
2. The laser of claim 1 wherein the mode cross-section of the
absorber section is wider than the mode cross-section of the gain
section.
3. The laser of claim 2 wherein the width of the mode cross-section
transitions smoothly from the gain section to the absorber
section.
4. The laser of claim 2 further comprising: a tapered waveguide
that transitions from a first width in the gain section to a
second, wider width in the absorber section.
5. The laser of claim 2 further comprising: a tapered ridge
waveguide that transitions from a first width in the gain section
to a second, wider width in the absorber section.
6. The laser of claim 1 wherein the mode cross-section of the
absorber section has a greater height than the mode cross-section
of the gain section.
7. The laser of claim 1 wherein: the gain section includes an
electrical contact for forward biasing the quantum-confined active
region; the absorber section includes an electrical contact for
reverse biasing the quantum-confined active region; and the gain
section and the absorber section are a single monolithic structure
but the gain section is electrically isolated from the absorber
section.
8. The laser of claim 7 further comprising: a proton-implanted
barrier located between the gain section and the absorber section
for electrically isolating the gain section from the absorber
section.
9. The laser of claim 7 further comprising: lower cladding
layer(s), lower waveguide layer(s), quantum-confined active region
layer(s) that form the quantum-confined active region, upper
waveguide layer(s) and upper cladding layer(s); wherein the gain
section includes the a first portion of the foregoing layers and
the absorber section includes a second portion of the foregoing
layers.
10. The laser of claim 1 wherein the integrated mode-locked
semiconductor laser is passively mode-locked.
11. The laser of claim 10 wherein saturation of the
quantum-confined active region of the absorber section produces the
loss modulation.
12. The laser of claim 1 wherein the integrated mode-locked
semiconductor laser is actively mode-locked.
13. The laser of claim 12 wherein the gain section further
comprises: an electrical contact for applying a periodically
modulated electrical signal to forward bias the quantum-confined
active region of the gain section, thus producing the loss
modulation.
14. The laser of claim 12 further comprising: a second gain section
including an electrical contact and a third portion of the
quantum-confined active region, the electrical contact for forward
biasing the quantum-confined active region of the second gain
section.
15. The laser of claim 12 wherein the absorber section further
comprises: an electrical contact for applying a periodically
modulated electrical signal to reverse bias the quantum-confined
active region of the absorber section, thus producing the loss
modulation.
16. The laser of claim 1 wherein the horizontal laser cavity
comprises two parallel planar mirrors.
17. The laser of claim 16 wherein the horizontal laser cavity
comprises a semiconductor structure cleaved on two ends to form two
parallel planar mirrors.
18. The laser of claim 17 wherein the two cleaved ends are coated
with dielectric reflection coatings.
19. The laser of claim 1 wherein the quantum-confined active region
comprises quantum well layers.
20. The laser of claim 1 wherein the quantum-confined active region
comprises quantum wires.
21. The laser of claim 1 wherein the quantum-confined active region
comprises quantum dots.
22. The laser of claim 21 wherein the semiconductor substrate is a
GaAs substrate, and the quantum-confined active region comprises
self-assembled InAs quantum dots in InGaAs quantum wells.
23. The laser of claim 1 wherein the substrate is a GaAs
substrate.
24. The laser of claim 1 wherein the substrate is an InP
substrate.
25. The laser of claim 1 wherein the substrate is a GaSb
substrate.
26. The laser of claim 1 wherein the substrate is a GaN
substrate.
27. The laser of claim 1 wherein the quantum-confined active region
is constructed from the InGaAs materials system.
28. The laser of claim 1 wherein the quantum-confined active region
is constructed from a materials system using at least two of the
following elements: In, Ga, As, P, Al.
29. The laser of claim 1 wherein the quantum-confined active region
is constructed from a materials system using Sb and at least one of
the following elements: In, Ga, As, P, Al.
30. The laser of claim 1 wherein the integrated mode-locked
semiconductor laser produces laser pulses in the 1060-1340 nm
wavelength range.
31. A device for producing laser pulses, comprising: a
semiconductor substrate; and a mode-locked semiconductor laser
integrated on the semiconductor substrate, the mode-locked
semiconductor laser comprising: a laser cavity having an optical
path; a gain section located along the optical path; an absorber
section location along the optical path, wherein a mode cross
section of the absorber section is larger than a mode cross section
of the gain section; and a quantum-confined active region located
in the gain section and/or the absorber section.
32. The device of claim 31 wherein a mode cross-section of the
absorber section has a larger area than a mode cross-section of the
gain section.
33. The device of claim 31 wherein the mode cross-section of the
absorber section is wider than the mode cross-section of the gain
section.
34. The device of claim 33 further comprising: a tapered waveguide
that transitions from a first width in the gain section to a
second, wider width in the absorber section.
35. The device of claim 31 wherein: the gain section includes an
electrical contact for forward biasing the quantum-confined active
region; the absorber section includes an electrical contact for
reverse biasing the quantum-confined active region; and the gain
section and the absorber section are a single monolithic structure
but the gain section is electrically isolated from the absorber
section.
36. The device of claim 35 further comprising: lower cladding
layer(s), lower waveguide layer(s), quantum-confined active region
layer(s) that form the quantum-confined active region, upper
waveguide layer(s) and upper cladding layer(s); wherein the gain
section includes the a first portion of the foregoing layers and
the absorber section includes a second portion of the foregoing
layers.
37. The device of claim 31 wherein the integrated mode-locked
semiconductor laser is passively mode-locked.
38. The device of claim 31 wherein the integrated mode-locked
semiconductor laser is actively mode-locked.
39. The device of claim 31 wherein the horizontal laser cavity
comprises two parallel planar mirrors.
40. The device of claim 31 wherein the quantum-confined active
region comprises quantum dots.
Description
CROSS-REFERENCE TO RELATED APPLICATION(S)
[0001] This application claims priority under 35 U.S.C. .sctn.
119(e) to U.S. Provisional Patent Application Ser. No. 60/662,451,
"High Power and Wide Operating Temperature Range Mode-Locked
Semiconductor Lasers," filed Mar. 15, 2005; and under U.S.
Provisional Patent Application Ser. No. 60/723,412, "High Power
Mode-Locked Semiconductor Lasers," filed Oct. 3, 2005. The subject
matter of all of the foregoing is incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] This invention relates to mode-locked semiconductor lasers
with a quantum-confined active region. 2. Description of the
Related Art
[0004] Laser mode-locking is a technique of generating optical
pulses by modulation of a resonant laser cavity. The laser cavity
includes a light-amplifying gain section, where population
inversion and positive optical feedback take place. The laser
cavity may also include an absorber section, where optical loss
takes place. Modulation of the gain and/or loss in these sections
(typically referred to as "loss modulation" regardless of whether
gain or loss is modulated) causes the laser light to collect in
short pulses located around the point of minimum loss. The pulses
typically have a pulse-to-pulse spacing given by the cavity
round-trip time T.sub.R=2L/v.sub.g, where L is the length of the
laser cavity (assuming a linear cavity) and v.sub.g is the group or
propagation velocity of the peak of the pulse intensity inside the
laser cavity.
[0005] For monolithic semiconductor lasers, two parallel and partly
transparent mirrors can be made by cleaving the semiconductor along
its crystallographic planes, thus forming a Fabry-Perot laser
cavity. Optical gain can be created by pumping (either electrically
or optically) an active region within the laser cavity. Active
regions can be based on conventional doped p-n junctions.
Alternately, active regions can be based on quantum-confined
structures, such as quantum wells, quantum wires and quantum dots.
Quantum-confined active regions have certain performance advantages
over more conventional p-n junction active regions. However, in
quantum-confined mode-locked semiconductor lasers, mode-locking
typically occurs for values of the pump current that are close to
its threshold value. This limits the maximum peak power that can be
achieved which, in turn, limits the possible applications for these
devices.
[0006] Thus, there is a need for quantum-confined mode-locked
semiconductor lasers that can achieve higher peak powers.
SUMMARY OF THE INVENTION
[0007] The present invention overcomes the limitations of the prior
art by providing a quantum-confined mode-locked semiconductor laser
in which the "mode size" of an absorption region in the laser
cavity is increased relative to the mode size of the gain region in
the laser cavity. In more detail, the semiconductor laser includes
a laser cavity with an optical path. A gain section and an absorber
section are located along the optical path and produce loss
modulation leading to the mode-locked behavior. The gain section
and/or the absorber section contain a quantum-confined active
region. The mode volume of the absorber section is increased (e.g.,
in length and/or cross-sectional area), thus reducing the optical
power density in the absorber section. This, in turn, delays
saturation of the absorber section until higher optical powers,
thus increasing the peak power that can be output by the laser.
[0008] In one design, the semiconductor laser includes a horizontal
laser cavity integrated on a semiconductor substrate. For example,
the laser cavity may be formed by cleaving two ends of a
semiconductor structure to form two parallel planar mirrors. The
mirrors may optionally be coated to achieve the desired
reflectivity. A quantum-confined active region is located along the
optical path of the laser cavity. For example, various epitaxial
layers may be grown on the substrate to form the quantum-confined
active region. One section of the quantum-confined active region is
used as part of the gain section, for example by forward biasing
that section of the quantum-confined active region. A different
section of the quantum-confined active region is used as part of
the absorber section, for example by reverse biasing this
section.
[0009] The gain section and absorber section are designed so that
the mode cross-section of the absorber section has a larger area
than the mode cross-section of the gain section. In one particular
design, the optical mode is laterally confined by a ridge
waveguide, which has a narrower width in the gain section and
flares out to a broader width in the absorber section. Other
waveguide designs can also expand in width to achieve a greater
mode cross-section in the absorber section than in the gain
section. The mode cross-section can also be expanded in the
vertical direction, for example by changing the size, spacing
and/or composition of the layers in the absorber section compared
to the gain section.
[0010] The principles described above can be applied to both
actively and passively mode-locked lasers. In one class of
passively mode-locked lasers, the gain and absorber sections are DC
biased and the saturation of the quantum-confined active region in
the absorber section creates the loss modulation that leads to
mode-locking. In one class of actively mode-locked lasers, a
periodically modulated electrical signal is applied to the gain
section and/or the absorber section, thus creating the loss
modulation.
[0011] The quantum-confined active region itself can have different
structures. Quantum wells, wire and dots are examples of
quantum-confined structures suitable for use in active regions.
Quantum dots are generally preferred due to their singular,
delta-function like density of states. In one design, the
semiconductor substrate is a GaAs substrate, and the
quantum-confined active region is based on self-assembled InAs
quantum dots in InGaAs quantum wells.
[0012] Other aspects of the invention include products based on the
structures described above, applications for these structures and
products, and methods for using and fabricating all of the
foregoing.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] The invention has other advantages and features which will
be more readily apparent from the following detailed description of
the invention and the appended claims, when taken in conjunction
with the accompanying drawings, in which:
[0014] FIG. 1 is a perspective diagram of a mode-locked
semiconductor laser according to the present invention.
[0015] FIG. 2 is a side cross-section of a three-section actively
mode-locked semiconductor laser.
[0016] FIG. 3 is a side cross-section of a two-section passively
mode-locked semiconductor laser.
[0017] FIG. 4 is a top view of a mode-locked semiconductor laser
using a tapered ridge waveguide.
[0018] FIG. 5 is a schematic of the distribution of the optical
field in the laser waveguide and cladding layer.
[0019] FIGS. 6A-6E are diagrams of epitaxial layer designs for
different semiconductor mode-locked lasers.
[0020] The figures depict embodiments of the present invention for
purposes of illustration only. One skilled in the art will readily
recognize from the following discussion that alternative
embodiments of the structures and methods illustrated herein may be
employed without departing from the principles of the invention
described herein.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0021] FIG. 1 is a diagram of an integrated mode-locked
semiconductor laser 100 according to the present invention. The
laser structure 100 is integrated onto the underlying substrate.
For example, it may be fabricated by epitaxially depositing
different layers of material onto the substrate. Alternately, it
may be fabricated by doping various regions of the substrate.
Etching and lithography are two common processes that may be used
to fabricate the integrated laser structure 100 on the
semiconductor substrate.
[0022] The laser 100 has a horizontal laser cavity 150. In this
example, the laser cavity 150 is a linear cavity defined by two
planar end mirrors 110A and 110B. The optical path 120 through the
laser cavity 150 is the round-trip path between the two mirrors
110.
[0023] For convenience, throughout this application, the x-y-z
coordinate system will be defmed with z being the direction of
propagation along the optical path 120, y being perpendicular to
the optical path 120 but parallel to the substrate surface, and x
being perpendicular to the substrate surface. The coordinate system
is defined locally for each point along the optical path. The y and
z directions may change if the optical path is not linear. Terms
such as "up," "down" and "vertical" refer to the x direction (i.e.,
generally perpendicular to the substrate surface), "lateral" refers
to the y direction, and "horizontal" generally means parallel to
the substrate surface. "Transverse," when referring to the optical
mode or optical propagation, refers to the x and y directions,
whereas "longitudinal" refers to the z direction. "Height" or
"thickness," "width," and "length" refer to quantities along the x,
y, and z directions, respectively.
[0024] The laser 100 also includes a gain section 160 and an
absorber section 170 located along the optical path 120. At least
one of the gain section 160 and the absorber section 170 also
includes a quantum-confined active region 180, such as quantum well
layers, quantum wires and/or quantum dots. Quantum wells are
structures having energy barriers that provide quantum confinement
of electrons and holes in one dimension, which is selected to be
less than the room temperature thermal de Broglie wavelength.
Quantum wires have energy barriers that provide quantum confinement
of electrons and holes in two dimensions, which are selected so
that each one is less than the room temperature thermal de Broglie
wavelength. Quantum dots have energy barriers that provide quantum
confinement of electrons and holes in all three dimensions, which
are selected so that each one is less than the room temperature
thermal de Broglie wavelength. Combinations of these structures can
also be used. For an electrically activated, quantum-confined gain
section 160, electrical energy is input to the quantum-confined
active region 180, which then amplifies light propagating through
the active region. For an electrically activated, quantum-confined
absorber section 170, energy from light propagating through the
quantum-confined active region 180 is converted from optical to
electrical form, thus introducing an optical loss in the optical
path.
[0025] The gain section 160 and/or absorber section 170 introduce a
loss modulation to light propagating around the laser cavity,
resulting in the collection of light into pulses that are emitted
by the laser 100 through one of the end mirrors 110. Various
examples of loss modulation are discussed in further detail
below.
[0026] The two end mirrors 110 help determine the longitudinal
optical characteristics of the laser cavity 150. The transverse
characteristics of the laser cavity 150 typically are determined by
waveguiding structures that help to laterally confine the light in
both the x and y directions as the light propagates around the
laser cavity. The waveguiding structures can vary along the optical
path, thus producing different transverse optical confinement at
different locations in the laser cavity. Different waveguide
designs at different points along the optical path can support
different transverse optical modes.
[0027] FIG. 1 shows cross-sections A-A and B-B of the laser cavity
within the gain section 160 and the absorber section 170,
respectively. The ovals 165 and 175 shown in these cross-sections
are a measure of the transverse optical confinement at each of
these cross-sections and will be referred to as the mode
cross-section. In one definition, the mode cross-section is defined
by the contour where the intensity is equal to 1/e times the peak
intensity of the optical mode at that cross-section (i.e., the
"near field"). The mode cross-sectional area is the area within the
contour, where the intensity is greater than the 1/e intensity. The
mode cross-section may include two or more disjoint areas,
depending on the intensity distribution of the optical mode.
[0028] In FIG. 1, the mode cross-section 175 of the absorber
section 170 has a larger area than the mode cross-section 165 of
the gain section 160. For a laser pulse of a given power, the
optical power density (i.e., optical power divided by the area of
the mode cross-section) will be reduced compared to an absorber
section 170 that has the same mode cross-section 165 as the gain
section 160. As a result, the absorber section 170 with larger mode
cross-sectional area will not saturate until higher pulse powers
are reached, thus allowing the laser to output higher power
pulses.
[0029] Monolithic mode-locked semiconductor lasers such as shown in
FIG. 1 offer significant advantages compared to other types of
mode-locked lasers (e.g., solid state mode-locked lasers, such as
Ti:sapphire or Nd:glass lasers) due to their compact size, inherent
reliability and suitability to be produced in significant volumes
by employing commercial high-yield manufacturing processes. They
are strong candidates for applications requiring a low-cost
reliable source of multi-GHz optical pulses to address high-volume
consumer applications, such as processor-to-processor and
on-processor optical clock distribution. Various embodiments of
these lasers can exhibit high output optical power and stable
performance in terms of pulsewidth, rms timing jitter, emission
wavelength, and pulse repetition frequency, often across a wide
operating temperature range.
[0030] FIGS. 2-3 are diagrams that illustrate different types of
mode-locking. FIG. 2 shows a three-section actively mode-locked
semiconductor laser. The laser in FIG. 2 has a gain section 260 and
an absorber section 270. The gain section 260 itself has two
sections 262 and 266. A common quantum-confined active region 280
runs through all three sections. Electrical contacts 263, 267 and
279 make the electrical connections to each of the three sections.
The first gain section 262 is driven by an electrical modulation
pulse that has a frequency which is an integral multiple of the
inverse of the cavity round-trip time. That is, if the cavity round
trip time is T.sub.R, then the period of the electrical modulation
pulse is T.sub.R/J for an integer J, which may be one. In this way,
each point of the light beam propagating around the laser cavity
experiences the same gain and absorption on each round trip, even
though that gain and absorption may be different from one point of
the light beam to the next. This creates the loss modulation (in
this case, an active modulation of the gain section) that leads to
mode-locking and pulse generation. The second gain section 267 is
forward biased with a DC current to provide steady gain for the
device. The saturable absorption region 279 is reverse-biased.
Other methods of active mode-locking may also be used. For example,
electrical modulation may be applied to the absorber section
instead of, or in synchronization with, the gain section.
[0031] In one class of actively mode-locked lasers, an
electronically driven loss modulation produces a sinusoidal loss
modulation with a period given by the cavity round trip time
T.sub.R. The saturated gain at steady state supports net gain
around the minimum of the loss modulation and therefore supports
pulses that are significantly shorter than the cavity round trip
time.
[0032] FIG. 3 is a schematic diagram of a two-section passively
mode-locked semiconductor laser. In this example, the gain section
360 is forward biased with a DC current to provide the overall gain
for the device and the saturable absorption region 370 is
reverse-biased. The saturable absorber is used to obtain a
self-amplitude modulation of light inside the laser cavity. The
saturable absorber introduces a loss that is a larger percentage
loss for low intensity light but a lower percentage loss for higher
intensity light due to saturation of the absorption process. Thus,
a short pulse produces a loss modulation, because the high
intensity at the peak of the pulse saturates the absorber more
strongly than its low intensity wings. The loss modulation
typically exhibits fast initial loss saturation (i.e. reduction of
loss) determined by the pulse duration and typically a somewhat
slower recovery depending on the detailed mechanism of carrier
dynamics in the saturable absorber and the applied reverse bias in
the absorption section.
[0033] The saturable absorbers currently used in semiconductor
lasers typically exhibit an absorption recovery time on the order
of a few tens of ps. E.g., see D. J. Derickson et. al., "Short
Pulse Generation Using Multisegment Mode-Locked Semiconductor
Lasers," IEEE Journal of Quantum Electronics, Vol. 28 (10), pp.
2186-2202 (1992). This fast recovery time results in a fast loss
modulation, which in turn generally allows shorter pulses.
Additionally, because the absorption recovery time limits the
achievable repetition rate in a passively mode-locked laser, an
absorption recovery time on the order of a few tens of ps implies
that a pulse repetition frequency on the order of 100 GHz is
possible. Experimentally, monolithic semiconductor lasers have been
passively mode-locked with repetition rates of 350 GHz. E.g., see
Y. K. Chen, et. al., "Subpicosecond monolithic colliding pulse
mode-locked multiple quantum well lasers," Applied Physics Letters,
Vol. 58, pp. 1253-1255 (1991).
[0034] The absorption of the saturable absorber preferably
saturates at a lower energy than the gain of the gain medium. The
saturation energy of a material is defined as:
E.sub.sat=h.nu.A/(.differential.g/.differential.N), (1) where h is
the Plank's constant, .nu. is the optical frequency, A is the mode
cross-sectional area inside the laser cavity, and
.differential.g/.differential.N is the differential gain with
respect to carrier density. The saturation energy is a measure of
the energy required to saturate the gain of the gain section or the
absorption of the absorber section. In semiconductor laser
materials, the slope of the gain versus carrier density function
typically decreases in value as the carrier density level is
increased. E.g., see G. P. Agrawal and N. K. Dutta, Semiconductor
Lasers, New York, Van Nostrand Reinhold, 1993. Because the carrier
density level in saturable absorbers is smaller than in gain
regions, semiconductor saturable absorbers typically have lower
saturation energies than semiconductor gain regions.
[0035] Furthermore, in analysis conducted by and results obtained
by the inventors, it appears that in mode-locked semiconductor
lasers with straight ridge waveguides the mode-locked peak power is
limited primarily by the size of the absorber section. In order to
generate mode-locked laser pulses with narrow pulse width and high
peak power, two design considerations are preferably followed.
First, the saturation energy of the absorber section preferably
should be lower than the saturation energy of the gain section,
based on the definition of Eqn. 1. This is typically the case in
semiconductor lasers as described above. Second, the maximum
achievable mode-locked peak power is typically proportional to the
power required to saturate the absorption of the absorber section
and obtain maximum carrier inversion.
[0036] Therefore, generally speaking, when the size of the absorber
section is increased, more power is required to saturate the
absorption in the absorber section. This, in turn, will extend the
mode-locking regime to larger values of the gain section pump
current with correspondingly higher output power. Put in another
way, increasing the volume of the optical mode (i.e., the mode
volume), and correspondingly decreasing the photon density, in the
absorber section generally means that more power will be required
to saturate the absorption in the absorber section and realize
efficient mode-locking. This will extend the mode-locking regime to
larger values of the gain section pump current with correspondingly
higher output optical power.
[0037] In one approach, the mode volume of the absorber section is
increased by increasing the length of the absorber section.
However, there is a limit to this approach, the preferred
acceptable length of the absorber section that leads to efficient
mode-locking depends on the particular technical specifications
such as target pulse duration, pulse repetition rate, and the
mechanism of the absorption process in the saturable absorber (e.g.
carrier recovery time). Under conditions of strong excitation, the
absorption in the absorber section is typically saturated because
the initial carrier states in the valence band are depleted while
the final carrier states in the conduction band are partially
occupied. Within a sub-ps timescale after the excitation, the
carriers in each band thermalize and this leads to a partial
recovery of the absorption. On a longer time scale, typically a few
ps to a few tens of ps in semiconductor materials, the carriers
will be removed by recombination and trapping, and absorption will
recover. Therefore, if the length of the absorber section exceeds a
certain limit, the pulse will be re-absorbed strongly and
mode-locking will be destroyed or the mode-locking characteristics
of the pulse will be degraded.
[0038] Therefore, the length of the absorber section typically is
bounded by various requirements. The absorber section generally
cannot be shorter than a certain length because a minimum level of
absorption is required in order to achieve mode locking with an
acceptably narrow pulse width. The maximum acceptable pulse width
typically is set by the requirements of the particular application.
In addition, various factors may limit the maximum length of the
absorber section. First, the absorption saturation energy in the
absorber section must be less than the gain saturation energy in
the gain section, thus limiting the maximum length of the absorber
section. Second, the absorber section cannot be too long or the
recovery of absorption may cause the laser to exceed limits for
certain characteristics of the laser pulse, such as pulse width and
jitter. Therefore the optimum length of the absorber section is
bounded by these upper and lower limits, although the specific
values for these upper and lower limits depend on the requirements
for the particular application (e.g. pulse width) and on the design
of the laser epi structure (which determines the gain, etc).
[0039] The design of the absorber section can be optimized not only
in length (i.e., along the z dimension), by selecting the
appropriate ratio of the length of the gain section to the length
of the absorber section, but also along one or more transverse
dimensions, such as along the lateral y dimension and/or the
vertical x dimension.
[0040] FIG. 4 is a top view of a mode-locked semiconductor laser
illustrating one example of this approach. In this example, a ridge
waveguide 430 is used for lateral optical confinement (i.e., in the
y direction) and the design of the epitaxial layers used to form
the laser are used for vertical optical confinement (i.e., in the x
direction). The ridge waveguide 430 is tapered, increasing to a
larger width in the absorber section 470. If all else is equal, the
mode cross-section of the absorber section 470 will be wider and
have a greater area than that of the gain section 460.
[0041] In more detail, the parameters L.sub.g and L.sub.a denote
the length of the gain and absorber section, respectively. In the
lateral direction, the ridge waveguide 430 has three sections: a
straight ridge waveguide section of width w.sub.1, and length
L.sub.1, a straight ridge waveguide section of width w.sub.2, (with
w.sub.2>w.sub.1) and length L.sub.3, and a flared or tapered
waveguide section of length L.sub.2 connecting the two straight
ridge waveguide sections and tapering from the narrow straight
waveguide (of width w.sub.1) towards the wider ridge waveguide (of
width w.sub.2). The tapered waveguide section is flared towards the
absorber section 470 of the mode-locked laser. In this example, the
laser pulses are output through the output facet of the gain
section (i.e., the lefthand side of the structure.
[0042] The boundary between the gain section 460 and absorber
section 470 of the mode-locked laser may be located anywhere within
the three waveguide sections. In FIG. 4, the boundary between the
gain and absorber sections is shown as occurring in the middle
waveguide section. The device layout is designed so that when
w.sub.2>w.sub.1, the mode width of the absorber section is
larger than the mode width of the gain section, which in turn
increases the power required to saturate the absorption in the
absorber section (compared to the case when W.sub.2=w.sub.1) and
therefore will extend the mode-locking regime to larger values of
the gain section pump current and in turn result in higher output
optical power. The upper limit to the width of the waveguide in the
absorber section typically is set by the requirement on the optical
mode to retain good spatial coherence and to avoid
filamentation.
[0043] In the vertical x direction, increases in the peak
mode-locked power can be similarly achieved by increasing the
height of the mode cross-section. For epitaxially grown devices,
this can be achieved by the design (thickness, composition, doping
level etc.) of waveguiding and/or cladding layers so as to expand
the optical mode in the vertical direction. Increased mode height
can increase the power required to saturate the absorption in the
absorber section and therefore can extend the mode-locking regime
to larger values of the gain section pump current and in turn
result in higher output optical power, whereas at the same time
maintaining the desired optical pulse characteristics, such as
jitter and pulse width. The peak mode-locked power can be improved
further increasing in the mode cross-sectional area in both the
lateral and vertical directions.
[0044] Increasing the mode cross-sectional area preferably is done
while taking account of other design factors. For example, the
optical confinement factor .GAMMA. and modal gain
(g.sub.m=.GAMMA.g.sub.0, where g.sub.0 is the material gain) should
be maintained at levels sufficient to support lasing. The optical
confinement factor is defined as the overlap of optical field and
the active gain material (whether bulk semiconductor, quantum well,
quantum wire, or quantum dot) and is given by .GAMMA. = .intg. x n
- d n / 2 x n + d n / 2 .times. .PSI. * .function. ( x , y ) .PSI.
.function. ( x , y ) .times. d x .times. d y .intg. - .infin.
.infin. .times. .PSI. * .function. ( x , y ) .PSI. .function. ( x ,
y ) .times. .times. d x .times. d y ( 2 ) ##EQU1## where x.sub.n,
d.sub.n denote the center position and the thickness of the
n.sup.th layer of the active gain material as shown in FIG. 5, the
summation of the top term is over all layers with active gain
materials, and .PSI.(x, y) is the wavefunction of the optical
field. In the configuration of FIG. 5, the optical field
distribution is determined mainly by the index of the cladding
layers, the index of the waveguide, and the height of the
waveguide. For example, the expansion of the optical field in the
vertical direction can be achieved by reducing the difference in
the refractive indices of the cladding and waveguide layers or by
reducing the height of the waveguide layer. As the optical field
expands, the confinement factor .theta. and the modal gain
(g.sub.m=.theta.g.sub.0) decrease. The lasing condition,
(.theta.g.sub.0-.alpha.)*L=0 sets a limit for the optical field
expansion, where .alpha. denotes the total losses of the laser
including waveguide and mirror losses. Increasing material gain and
reducing internal loss and waveguide loss enable further expansion
of the optical field and therefore higher mode-locked peak
power.
[0045] Different types of quantum-confined active regions can be
used, including quantum wells, quantum wires and quantum dots.
However, in contrast to quantum wells, where carriers are localized
and confined in one dimension, and quantum wires, where carriers
are localized in two dimensions, quantum dots confine the electrons
or holes in all three dimensions and, thus, exhibit a discrete
energy spectrum. Such three-dimensional carrier confinement, which
leads to singular, delta-function like, density of states, sharp
electronic transitions and a pure optical spectrum, result in
certain advantages for quantum dot mode-locked lasers compared even
to quantum well and quantum wire mode-locked lasers.
[0046] For example, passively mode-locked quantum dot lasers can
exhibit low rms timing jitter, which can eliminate the need for
more expensive and complicated active or hybrid mode-locking
schemes. The timing jitter in passively mode-locked lasers
typically arises from fluctuations in the carrier density, photon
density, and index of refraction caused by amplified spontaneous
emission. Due to the discrete energy levels and low transparency
current in a quantum dot active gain region, the portion of
carriers involved in non-stimulated emission is significantly
reduced, resulting in a low value of the linewidth enhancement
factor and in turn low timing jitter.
[0047] The linewidth enhancement factor a describes the degree to
which variations in the carrier density N alter the index of
refraction n of an active layer for a particular gain g at the
lasing wavelength .lamda.. The linewidth enhancement factor can be
mathematically expressed as:
.alpha.=(4.pi./.lamda.)[(dn/dN)/(dg/dN)] (3) Experiments indicate
that the linewidth enhancement factor of quantum dot lasers can
reach 0.1, which is almost twenty times lower than for comparable
quantum well lasers (e.g., see T. C. Newell et. al., "Gain and
linewidth enhancement factor in InAs quantum dot laser diodes,"
IEEE Photonics Technology Letters, Vol. 11(12), pp. 1527-1529
(1999)), as further described in U.S. Pat. No. 6,816,525, "Quantum
Dot Lasers," which is incorporated herein by reference. The low
linewidth enhancement factor correspondingly reduces the rms timing
jitter exhibited by the quantum dot mode-locked lasers. Operation
of passively mode-locked quantum dot lasers that exhibit rms timing
jitter less than 1 ps at a 5-GHz pulse repetition rate has been
demonstrated. See L. Zhang, et. al., "Low timing jitter, 5 GHz
optical pulses from monolithic two-section passively mode-locked
1250/1310 nm quantum dot lasers for high speed optical
interconnects," Paper OWM4, OFC/NFOEC 2005 Technical Conference,
Mar. 6-11, 2005, Anaheim, Calif. USA. This is more than one order
of magnitude lower than the rms timing jitter exhibited by
comparable quantum well lasers.
[0048] Quantum dot mode-locked lasers can also exhibit
insensitivity to external spurious feedback, generated, for
example, by coupling light from the laser into a fiber. Such
insensitivity to external feedback can be important when packaging
the devices because it eliminates the need for expensive
sub-components, such as optical isolators.
[0049] Quantum dot mode-locked lasers can also exhibit improved
performance in terms of threshold current and power slope
efficiency across a wide operating temperature range (e.g.,
0.degree. C. to 125.degree. C.), for example through optimization
of the structural properties of the quantum dots, specifically the
dot size uniformity or through the introduction of modulation
p-type doping in the active region. E.g., see D. G. Deppe, et. al.,
"Modulation characteristics of quantum dot lasers: the influence of
p-type doping and the electronic density of states on obtaining
high speed," IEEE Journal of Quantum Electronics, Vol. 38(12), pp.
1587-1593 (2002); and K. Mukai, et. al., "High characteristic
temperature of near 1.3-micron InGaAs/GaAs quantum dot lasers at
room temperature," Applied Physics Letters, Vol. 76(23), pp.
3349-3351 (2000).
[0050] Quantum dot lasers can also exhibit low internal losses
.alpha..sub.I, (not to be confused with the linewidth enhancement
factor .alpha. of Eqn. 3). This is important in order to obtain
low-jitter, high optical power passively mode-locked lasers.
Internal losses in semiconductor lasers are primarily contributed
by free carriers absorbed in the laser waveguide regions. In
quantum dot lasers, such as those described in U.S. Pat. No.
6,816,525, "Quantum Dot Lasers," as the localization of the active
region gets deeper, due to the incorporation of the quantum dots
inside a quantum well, the free carrier population in the GaAs
matrix (i.e., the waveguide layer) is reduced, leading to a
corresponding reduction in internal losses.
[0051] Additionally, an important manufacturing advantage is the
fact that quantum dot mode-locked lasers emitting within the
1060-1340 nm wavelength range can be grown on GaAs substrates,
which leads to significantly higher manufacturing yields compared
to quantum well lasers emitting within the similar wavelength range
but grown instead on InP substrates.
[0052] FIGS. 6A-6E show the epitaxial structures of selected
embodiments of quantum dot passively mode-locked lasers. These
lasers under passive mode-locking operation have demonstrated high
peak mode-locked power (larger than 1 W), low timing jitter (less
than 10 fs pulse-to-pulse jitter), and narrow pulses (less than 10
ps pulse width) across the 30-60.degree. C. temperature range. In
certain designs, the length of the absorber section is between 1/20
to 1/5 of the total length of the laser cavity, the width of the
waveguide of the absorber section varies between 3 and 11 .mu.m,
and the ratio of the width of the waveguide in the absorber section
to the width of the waveguide in the gain section varies between
1:1 and 4:1. The total length of the laser cavity is determined in
part by the pulse repetition rate. For a pulse repetition rate with
period T.sub.P, the cavity round trip time preferably is T.sub.R=J
T.sub.P where J is a non-zero integer. The cavity round trip time
is, in turn, determined by the total length of the laser cavity.
For certain applications, the mode-locked laser is designed for a
pulse repetition rate of between 5-100 GHz.
[0053] The epitaxial structures shown can be used in a number of
structures with different vertical and lateral characteristics. In
one approach, the layers are epitaxially grown on the substrate and
then laterally patterned by subsequent etching, resulting in a mesa
structure as shown in FIG. 1. In a different design, the layers are
epitaxially grown but they are not laterally patterned. Rather,
they remain buried. Lateral confinement of the optical mode can be
achieved by etching isolation trenches, doping to achieve optical
confinement or by use of a ridge waveguide (as shown in FIG.
4.)
[0054] The active region in these examples is self-assembled InAs
quantum dots formed in InGaAs quantum wells that are grown on a
GaAs substrate by molecular beam epitaxy, based on epitaxial growth
techniques and designs as described in U.S. Pat. No. 6,816,525,
"Quantum Dot Lasers," which is incorporated herein by reference. In
the case of an ideal quantum dot array, i.e. quantum dot structures
having a delta-function-like density of states, the operating
temperature will not significantly adversely affect the performance
characteristics of a quantum dot mode-locked laser. One difference
that distinguishes realistic lasers based on a self-organized
quantum dot array from the ideal case is the inhomogeneous
broadening of the energy levels due to the size fluctuation of
quantum dots. The structural properties (i.e. shape, size and
surface density) of self-assembled quantum dots formed via the
Stranski-Krastonow method depend on the growth conditions, such as
the growth temperature of the active region and surrounding
semiconductor matrix (barriers, cladding layers), the composition
of surrounding structures including the strain of the underlying
quantum well, the design parameters of the active region (e.g.
thickness of quantum wells and barriers), the material growth
rates, and the arsenic overpressure among others.
[0055] In order to achieve a more uniform quantum dot size
distribution within a stack and from stack-to-stack in quantum dot
mode-locked lasers, the design of the epitaxial structure of the
laser is preferably optimized for example through appropriate
adjustment of the number of quantum dot stacks, the thickness of
the quantum wells and the barrier layers in the laser active
region.
[0056] FIG. 6A shows one embodiment of a laser epitaxial design
which can be used for quantum dot passively mode-locked lasers
based on the principles described above. It is an illustration of a
growth sequence for a laser having six layers of InAs quantum dots
grown within and surrounded by an In.sub.0.15Ga.sub.0.85As quantum
well. The quantum well assists the quantum dots to capture and
retain injected carriers due to the lower bandgap energy of the
central quantum well layer compared with surrounding barrier
layers. An n-type GaAs buffer layer (#2) is grown on a GaAs
substrate (#1). An approximately two micron thick AlGaAs cladding
layer (#3, 4, 5) is then grown. This is followed by graded AlGaAs
layer (#6) and a GaAs waveguiding layer (#7), which are undoped to
reduce absorption losses.
[0057] The quantum-confined active region is composed of six
In.sub.0.15Ga.sub.0.85As quantum wells (#8) of approximately 7.6 nm
thickness. Inside each quantum well, InAs quantum dots of an
equivalent thickness equal to 2.4 monolayers have been grown, based
on the techniques described in U.S. Pat. No. 6,816,525, "Quantum
Dot Lasers," which are incorporated herein by reference. The
quantum wells are separated from each other by GaAs barriers (#9)
of approximate thickness 16 nm. In one embodiment, following the
growth of the quantum well and prior to the growth of the barrier,
several monolayers of GaAs are grown, followed by a growth
interruption step in which the substrate temperature is raised to
approximately 580-610.degree. C. The growth interruption step
preferably lasts long enough to desorb excess segregated indium
from the surface prior to commencing growth of the GaAs barrier
layer.
[0058] After the growth of the last InGaAs quantum well is
completed, a GaAs waveguiding layer (#10) and a graded AlGaAs layer
(#11) are grown, both undoped. An approximately two micron thick
upper AlGaAs cladding layer (#12, 13, 14) is then grown, followed
by a GaAs cap layer (#15). An electrical contact makes contact with
the cap layer.
[0059] Layers 7, 8, 9 and 10 form a waveguide core region having a
higher refractive index than the surrounding AlGaAs cladding
layers, with the upper cladding layer composed of layers 11, 12,
13, and 14 and the lower cladding layer composed of layers 3, 4, 5
and 6. Consequently, this structure confines the optical mode in
the vertical direction. A fraction of the optical mode will be
confined in the portion of the structure occupied by the quantum
dots.
[0060] Confinement in the lateral direction can be achieved by a
variety of approaches. For example, the structure shown in FIG. 6A
can be grown as a mesa (e.g., see FIG. 1), resulting in lateral
confinement of the optical mode. Alternately, the layers shown in
FIG. 6A need not extend indefinitely in the lateral direction. The
layers can be lithographically defined to have a finite lateral
extent, and then surrounded by lower index materials to form a
lateral waveguide structure. As a final example, a ridge can be
added to the structure shown in FIG. 6A to produce a lateral
waveguiding effect.
[0061] FIGS. 6B-6E show additional exemplary embodiments of laser
epitaxial designs which can be used for either quantum dot
passively mode-locked lasers or quantum dot actively mode-locked
lasers. Electrically, the p-type layers, undoped layers and n-type
layers form a p-i-n structure. While one substrate polarity is
shown, the doping polarity of the layers may be reversed in other
embodiments from what is shown in these exemplary embodiments.
[0062] The quantum well layers in the active region (#8) provide a
means to improve carrier capture by the quantum dots and also serve
to reduce thermionic emission of carriers out of the dots. In a
quantum dot laser, the fill factor of quantum dots in an individual
quantum dot layer is low, typically less than 10%, depending upon
the dot density and mean dot size. Because the quantum dots are
disposed within the quantum well, carriers captured by the well
layer of the quantum well may be captured by the quantum dot,
thereby increasing the effective fill factor of quantum dots.
Additionally, the barrier layers of the quantum well serve to
reduce thermionic emission out of quantum dots.
[0063] The generation of ultra-fast optical pulses from monolithic
semiconductor lasers is attractive owing to the compact and
efficient properties of these devices. Applications of these
devices include but are not limited to optical time division
multiplexing, photonic switching, electro-optic sampling, optical
computing, optical clocking, applied nonlinear optics and other
areas of ultrafast laser technology.
[0064] While particular embodiments and applications of the present
invention have been illustrated and described, it is to be
understood that the invention is not limited to the precise
construction and components disclosed herein and that various
modifications, changes and variations which will be apparent to
those skilled in the art may be made in the arrangement, operation
and details of the method and apparatus of the present invention
disclosed herein without departing from the spirit and scope of the
invention as defined in this description.
[0065] For example, while embodiments of the present invention have
been discussed in detail with regards to quantum dot layers
comprising InAs embedded in InGaAs quantum wells, this invention
may be practiced in other compound semiconductor materials. For
example, InGaAs quantum wells may be replaced with AlInGaAs wells.
Similarly, the barrier layers may comprise a variety of materials,
such as AlGaAs or AlGaInAsP. It will be understood that the barrier
layers may be comprised of a material having a lattice constant
selected so that the barrier layers between quantum dot layers
serve as strain compensation layers. In addition to quantum dot
layers, in alternative embodiments, the active region may be
comprised of quantum wells, quantum wires or combinations
thereof.
[0066] The present invention has been discussed in detail in
regards to laser structures grown on GaAs substrates. GaAs
substrates have many advantages over other semiconductor
substrates, such as a comparatively larger wafer sizes and higher
manufacturing yields. However, embodiments of the present invention
may be practiced on other types of substrates, such as InP
substrates. Additionally, while molecular beam epitaxy has been
described as a preferred fabrication technique, it will be
understood that embodiments of the present invention may be
practiced using other epitaxial techniques alternatively or
additionally.
[0067] As a final example, in FIGS. 2-3, there is a single
underlying structure and active region which is used by both the
gain section and the absorber section. Forward biasing results in a
gain section; reverse biasing results in an absorber section. In
some embodiments, the isolation between adjacent gain sections is
provided by proton implantation, with an isolation resistance on
the order of several M.OMEGA.. One advantage of this approach is
that different sections can be fabricated at the same time using
the same semiconductor fabrication processes. However, this
approach is not the only possible approach. For example, different
sections of the laser could be fabricated at different times using
different processes. The sections could also be separated by air
gaps. For example, the gain section could be built up as one mesa
and the absorber section as a separate mesa. Other approaches will
be apparent.
* * * * *